The present invention relates to a functional film. In the present invention, the functional film is defined as follows. That is, the functional film means a film having a function, and the function means an action exhibited through a physical and/or chemical phenomenon. The functional film includes films having various functions such as a conductive film, a magnetic film, a ferromagnetic film, a dielectric film, a ferroelectric film, an electrochromic film, an electroluminescence film, an insulating film, a light-absorption film, a light-selective-absorption film, a reflection film, an antireflection film, a catalyst film, and a photocatalyst film.
In particular, the present invention relates to a transparent conductive film. The transparent conductive film can be used as a transparent electrode such as an electrode for an electroluminescence panel, an electrode for an electrochromic device, an electrode for a liquid crystal display, a transparent planar heating element, or a touch panel, and also as a transparent electromagnetic wave-shielding film.
Heretofore, functional films comprising a variety of functional materials have been produced by a physical vapor deposition (PVD) such as vacuum deposition, laser-abrasion, sputtering, or ion plating, or a chemical vapor deposition (CVD) such as thermal CVD, light CVD, or plasma CVD. These methods generally require huge facilities and some of them are not suitable for the formation of a film of a large area.
Furthermore, the film formation by coating using a sol-gel method is also known. The sol-gel method is also suitable for the formation of a film having a large area, but in most cases, it is necessary to sinter an inorganic material at a high temperature after the coating.
For example, the production of the transparent conductive film is as follows. Currently, the transparent conductive film is mainly produced by a sputtering method. There are a variety of sputtering procedures. One example is a method of forming a transparent conductive layer by acceleration-bombarding inert gas ions generated by direct current or high-frequency discharge in vacuum to a target surface, beating the atoms constituting the target out of the surface, and depositing them onto a support surface.
The sputtering method is advantageous because a transparent conductive film having a low surface electric resistance can be formed even when the film is large to some degree. However, the method has defects that the apparatus is large and the film formation is slow. When a transparent conductive film having a larger area is required in future, the apparatus should be larger. This requirement results in the necessity of enhanced accuracy on control as a technical problem, and also results in the problem of increase of production cost in another aspect. Furthermore, for compensating the slow film formation, the formation is accelerated by increasing the number of the target, but the increase is problematic because it also makes the apparatus larger.
The production of the transparent conductive film is also attempted by an coating method. A conventional coating method comprises applying a conductive coating composition, wherein conductive fine particles are dispersed in a binder solution, onto a resin film, and drying and hardening the composition to form a transparent conductive film. The coating method is advantageous because a transparent conductive film having a large area can be easily formed, the apparatus is simple, productivity is high, and the transparent conductive film can be produced at a cost lower than that in the sputtering method. In the transparent conductive film formed by the coating method, conductivity is expressed by the formation of an electric pathway owing to the mutual contact of the conductive fine particles. However, the transparent conductive film prepared by the conventional coating method has a defect that the contact of the conductive fine particles is insufficient owing to the presence of the binder and thus the resulting transparent conductive film has a high electric resistance (inferior conductivity), so that the use is limited.
As a production of a transparent conductive film by the conventional coating method, Japanese Patent Application Laid-Open No. 109259/1997 discloses a process for producing the film comprising a first step of forming a conductive layer by applying a coating composition comprising conductive powder and a binder resin onto a plastic film for transcription and drying the coated film, a second step of pressing the surface of the conductive layer to a smooth plane (5 to 100 kg/cm2) and heating the surface (70 to 180xc2x0 C.), and a third step of laminating the conductive layer on a plastic film or sheet and fixing them by applying pressure under heating.
In this method, a conductive film having a low electric resistance is not obtained because of the use of a large amount of the binder resin (100 to 500 parts by weight of conductive powder relative to 100 parts of the binder in the case of inorganic conductive powder; 0.1 to 30 parts by weight of conductive powder relative to 100 parts of the binder in the case of organic conductive powder).
Further, Japanese Patent Application Laid-Open No. 199096/1996 discloses a method of applying a coating composition for forming a transparent conductive film comprising tin-doped indium oxide (ITO) powder, a solvent, a coupling agent, and an organic or inorganic acid salt of a metal but containing no binder onto a glass plate, and sintering it at a temperature of 300xc2x0 C. or higher. In this method, the electric resistance of the conductive film is low because of no use of binder. However, since it is necessary to conduct the
sintering step at a temperature of 300xc2x0 C. or higher, it is difficult to form a conductive film on a support such as a resin film. That is, a resin film is melted, carbonized, or fired at the high temperature. The temperature limit may depend on the kind of the resin films and, for example, it may be 130xc2x0 C. for polyethylene terephthalate (PET) film.
As a conductive film formed by other method than the coating method, Japanese Patent Application Laid-Open No. 13785/1994 discloses a conductive film comprising a powder-compressed layer where at least part of, preferably all of the voids of skeleton structure constituted by conductive material (metal or alloy) powder are filled with a resin, and a resin layer present under the layer. The method of the production will be explained by exemplifying the case of forming a film on a plate material. According to the above patent publication, a resin, a powdery material (metal or alloy) and a plate material which is a member to be treated are first shaken or stirred in a vessel together with a film-forming medium (steel balls having a diameter of several millimeter) to form a resin layer on the surface of the member to be treated. Successively, the powdery material is trapped and fixed in the resin layer by the adhesive action of the resin layer. Further, the film-forming medium shaken or stirred imparts an impact force to the powdery material shaken or stirred to form a powder-compressed layer. However, for obtaining a fixing effect of the powder-compressed layer, a considerable amount of the resin is required. Moreover, the process is more complicated than the case of the coating method.
As another conductive film formed by a method other than the coating method, Japanese Patent Application Laid-Open No. 107195/1997 discloses a conductive fiber-resin integrated layer obtained by depositing a conductive short fiber on a film of PVC and the like through sprinkling the fiber, followed by pressurization. The conductive short fiber is a short fiber such as polyethylene terephthalate subjected to a covering treatment such as nickel plating. The pressurizing operation is, however, preferably conducted under a temperature condition at which the resin matrix layer shows thermoplasticity and thus, conditions of a high temperature and a low pressure such as 175xc2x0 C. and 20 kg/cm2 are required, so that it is difficult to form a conductive film on a support such as a resin film.
In consideration of such circumstances, it is desired to develop a functional film capable of forming easily a film having a large area, which can be prepared using a simple apparatus with high productivity and low cost, as well as has a high quality.
In particular, for a conductive film, it is desired to develop a conductive film capable of forming easily a film having a large area, which can be prepared using a simple apparatus with high productivity and low cost, as well as has a high quality.
An object of the present invention is to provide functional films which can exhibit a variety of functions using functional fine particles.
In particular, an object of the present invention is to provide a conductive film having a low resistance value using functional fine particles.
For achieving such an object, the functional film of the present invention is constituted by a functional film comprising a support and a functional layer on at least one surface of the support, wherein the above functional layer contains functional fine particles, and a ratio ("sgr"1/"sgr"2) between a dispersion value ("sgr"2) obtainable from the alignment of the functional fine particles at the front surface of the functional layer and a dispersion value ("sgr"1) obtainable from the alignment of the functional fine particles at the opposite surface of the functional layer is from 1.2 to 1.85.
As a preferred aspect of the functional film, it has a constitution wherein the above support is a transparent resin film.
As a preferred aspect of the functional film, it has a constitution wherein the above functional fine particles are conductive fine particles.
As a preferred aspect of the functional film, it has a constitution wherein average primary particle size of the above conductive fine particles is in the range of 5 to 50 nm.
As a preferred aspect of the functional film, it has a constitution wherein the thickness of the above functional layer is in the range of 0.5 to 5 xcexcm.
Further, as a preferred aspect of the functional film, it has a constitution wherein the above functional layer contains a resin in an amount of the range of 3.7 by volume or less when the volume of the above conductive fine particles is regarded as 100.
According to the present invention as above, the functional film comprising a functional layer on at least one surface of a support is a film wherein the above functional layer contains functional fine particles, and a ratio ("sgr"1/"sgr"2) between a dispersion value ("sgr"2) obtainable from the alignment of the functional fine particles at the front surface of the functional layer and a dispersion value ("sgr"1) obtainable from the alignment of the functional fine particles at the opposite surface of the functional layer is 1.2 or more, so that a sufficient contact of the functional fine particles is effected in the functional layer and therefore, the strength of the functional layer and the adhesiveness between the functional layer and the support become large. Accordingly, a transparent conductive film wherein conductive fine particles are used as the functional fine particles, for example, has a low electric resistance. Moreover, the adhesiveness between the support and the functional layer is strong enough to use it for a long period of time. Furthermore, it is also possible to use a transparent support such as a transparent resin film as the support, and the functional film of the present invention can be formed as a film having a large area by changing a coating apparatus or a compressing apparatus.